1. Field of the Invention
The present invention relates to an optical apparatus for scanning an inner face of a drum, and more particularly to an image recording apparatus of a drum-inner-face scanning type for recording an image on a recording medium such as a photosensitive film held on the inner face of a drum. The present invention also relates to a method of scanning an inner face of a drum.
2. Description of Related Art
Drum type image recording scanners include two types which differ on where to mount a photosensitive film. The first type holds a photosensitive film on the outer face of the drum on which light beams scan to record an image, and the second type holds a photosensitive film on the inner face of the drum.
In the first type, or the drum-outer-face scanning type, a recording head for emitting light beams is disposed outside the drum and the main scanning is performed by rotation of the drum. The drum-outer-face scanning type has an advantage of the ease with which plural light beams can be used for scanning. However, this first type has disadvantages of the relatively large size because of the recording head disposed outside the drum and of the need for relatively large power to rotate the drum.
In the second type, or the drum-inner-face scanning type, a recording head is disposed inside the drum and the main scanning can be performed by rotation of the recording head. An example of the drum-inner-face scanning type is disclosed in U.S. Pat. No. 4,853,709. The drum-inner-face scanning type has advantages of having a relatively small size and less need for power to rotate the recording head.
Multibeam scanning is preferable in order to increase the recording speed. However, the drum-inner-face scanning type has the following problem in multibeam scanning and thus, has not been used as much. FIG. 1 schematically illustrates the problem when the plural beams were used in the drum-inner-face scanning type. Plural light beams A2, A3 and A4 go along the axis of a drum A5, and are reflected at a mirror A1 to be incident on the inner face of the drum A5. The mirror A1 rotates to perform the scanning of the inner face of the drum by the plural beams. Disposition of the light beam spots on the inner face of the drum A5 are varied with the rotational position of the mirror A1, and eventually, the order of the plural light beams is inverted, for example, from (A2, A3, A4) to (A4, A3, A2) at opposite sides of the drum. Scanning lines of the plural light beams thus cross one another on the photosensitive film on the inner face of the drum, and this makes the multibeam scanning impossible in the image recording apparatus of the drum-inner-face scanning type.
The same problem is observed when a pentagonal prism is used, as in the U.S. Pat. No. 4,853,709, in place of the mirror A1.